Dustless propellent powder containing coated spherical nitrocellulose



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This invention relates to propellent explosives and particularly toimproved propellent powders and methods of making them.

Single or double base grains for use in rocket motors, missiles, and gasgenerating devices of various types, are quite large. For example, theycan be a foot or more in diameter and several feet in length. Suchgrains have been conventionally prepared by the methods employed in themanufacture of extruded powder. However, the curing and solvent removalof these relatively large grains have presented a formidable problem.These conventional manufacturing difiiculties are readily overcome whenthe large propellent or gas generating grains are prepared in accordancewith the teachings of U.S. Patent 2,916,775, issued to John J. ONeill,Jr. In the manufacture of large propellent grains by this process,finely divided particles of a nitrocellulose base propellent are mixedwith a plastisol solvent consisting of relatively non-volatilenitrocellulose plasticizers to form a flowable paste. The paste is thenformed into the shape of a rocket powder grain and maintained in theassumed shape until it becomes consolidated to form the desired rocketpowder grain.

Molding or casting processes of this type are particularly well adaptedfor the manufacture of large diameter propellent powder grains. However,the handling of the exceedingly small propellent powder particles usedin making up the large grains poses several inherent difiiculties. Thenitrocellulose propellent particles have an average size of about 0.003inch and because of this exceedingly small size, are quite dusty. Thispresents an intense safety problem due to the high concentration ofexplosive dust in the atmosphere, and also necessitates frequentcleaning of the operating area. In addition, the finely dividednitrocellulose particles are quite susceptible to the accumulation ofstatic charges. Thus, extreme care must be exercised to avoid thegeneration of a spark in this raw material. Also, the finely dividednitrocellulose particles are not readily wetted by the plastisolsolvent. Thus, mixing is necessarily prolonged because it must becontinued until all particles are uniformly wetted with the plastisolsolvent.

it is, therefore, an object of this invention to provide propellentpowders overcoming the disadvantages of the prior art. It is also anobject of this invention to provide propellent powder grains exhibitinglittle or no dusting while retaining their free-flowing characteristics.A further object of this invention is to provide an improved astaticpropellent powder charge. In addition, it is an object of this inventionto provide propellent powder grains having improved wettability by acasting solvent. A more specific object of this invention is to providean improved method for the manufacture of cast propellent powder grainsfor use in rocket motors and the like.

In accordance with this invention, these and other objects areaccomplished, generally speaking, by coating the grains of a propellentnitrocellulose-base powder charge with a substantially non-volatileliquid which is compatible with nitrocellulose and which has noappreciable plasticizing action on nitrocellulose. More specifically,this invention contemplates surface coating nitrocellulose propellentpowder grains with a substantially non-volatile liquid ester formed bythe reaction of @lli.

Patented Get. 29, 1953 2 phthalic acid or the dibasic saturatedaliphatic acids having between 4 and 8 carbon atoms in their alkyl chainwith saturated aliphatic alcohols having between 6 and 12 carbon atomsin their alkyl chain. In addition to phthalic acid, the acids that canbe employed in accordance with the present invention are adipic acid,pimelic acid, suberic acid, azelaic acid, and sebacic acid. Dioctylphthalate is the preferred ester employed in accordance with the presentinvention, but the objects of the present invention can also be realizedby using one or more of the di-esters of the acids and alcohols referredto above. Since the (ii-esters employed have no appreciable solventpower for nitrocellulose, they do not penetrate the surface of thegrains. They are present only as a molecular external coating.

In order to minimize the dusting and static characteristics of thenitrocellulose propellent powder grains while maintaining theirfree-flowing characteristics, the amount of the di-ester present on thesurface of the powder grains must be maintained between about 0.02% andabout 0.5% of the weght of the powder. Amounts of the diester less thanabout 0.02% have no appreciable elfect. On the other hand, when morethan approximately 0.5% of the di-ester is used as a coating agent, itdetracts from the free-flowing qualities of the powder and merely actsas a diluent for the propellent charge.

Nitrocellulose powders can be coated with the diesters of the presentinvention in any convenient manner. It is only necessary that the grainsmaking up the charge are substantially uniformly coated. Thus, thepropellent powder charge can be mixed with the liquid di-ester in asweetie barrel or other conventional tumbling apparatus to effect thecoating. This tumbling operation can be performed in the presence orabsence of water. H desired, the powder and/ or the coating agent can bemixed with water prior to the coating operation. It is preferred,however, to treat an aqueous slurry or suspension of the powder grainsto be coated with an aqueous emulsion or suspension of the liquiddi-ester in a continuous manner. This is most readily accomplished bydewatering an aqueous slurry of the powder grains in a continuouscentrifuge and then adding an aqueous emulsion of the coating materialto the centrifuge. The emulsified diester readily replaces the waterwetting the powder, and the excess emulsion is conveniently removed fromthe powder charge by continued centrifugation. Any water remaining onthe powder grains can then be removed by conventional drying methods.

The di-ester coatings of the present invention can be applied tonitrocellulose powder of any size or shape. However, they are moreadvantageously utilized with spherical propellent powder grains havingan average diameter of about 0.003 inch and used as the base mate'- rialin the manufacture of giant rocket grains and the like. Although thepresent invention is concerned primarily with the treatment of suchcasting powder, the qualities of standard small arms ammunitionpropellent are also materially improved when coated with the diester ofthe present invention. The present coating materials can be used toreplace or augment the conventional graphite glaze on commercial andmilitary propellents. Thus, the present invention is applicable to anytype of nitrocellulose based powder regardless of its size or shape.

When the propellent powder grains coated in accordance with thisinvention are used as a base material in the manufacture of rocketgrains, they are mixed with a plastisol solvent which has relativelypoor solvent power for the propellent at ordinary room temperature, butwhich is very rapid acting and has a relatively great solvent power atelevated temperatures. The plastisol solvent can be either a deterrentor an energizing modi- 3 fier or a mixture of energizers and deterrents.Suitable energizers include nitroglycerin, tri thyiene glycol dinitrate,butane triol trinitrate, diglycol dinitrate, ethylene glycol dinitrate,and the like. Suitable deterrents that may be used include dibutylphthalate, dimethyl sebacate, dibutyl succinate, dibutyl adipate,dipropyl adipate, triacetine, ethyl diphenol phosphate, tributylphosphate, and the like. The di-esters of the present invention arereadily compatible with plastisol solvents of this type and greatlyreduce the time required for obtaining a uniform plastisol mixture.Also, because the (ii-esters have no appreciable solvent power towardthe nitrocellulose propellant, the pot life of the plastisol mixture isnot lessened.

In order that the invention may be further clari following is an exampleillustrating a preferred embouiment thereof. In this embodiment andthroughout the specification, all proportions are expressed by weightunless otherwise indicated.

About 1000 parts of an aqueous powder slurry obtained in accordance withthe process of US. 2,888,713, granted to Eugene A. Andrew, was fed to acontinuous centrifuge. The powder was substantially spherical in formand had an average diameter of approximately 0.003 inch. The powderslurry was about 95% water and 5% powder. The water content of thepowder was reduced to approximately 8% by centrifuging. At this point,about 7 parts of an aqueous emulsion of dioctyl phthalate was graduallyadded to the centrifuge over about 5 minutes. This aqueous emulsionconsisted of about 1.15% dioctyl phthalate and about 98.85% water. Thisemulsion was run into the centrifuge and through the wet powder chargeuntil the original water content of the powder was replaced with theaqueous dioctyl phthalate emulsion. The powder was then removed from thecentrifuge and the water driven off of the powder by drying in a traydryer for about 10 hours at a temperature of approximately 65 C. At theend of the drying period, the powder was substantially free of water andcontained a uniform surface coating of dioctyl phthalate. The dioctylphthalate on the surface of die grain constituted about 0.1% of theweight of the powder. This powder charge was completely dust-free andsubstantially astatic. Also, the coating had no deleterious effects onthe free-flowing characteristics of the powder charge. This material wasparticularly well adapted for the manufacture of rocket grains by theplastisol process because of its improved wettability by the castingsolvents.

Nitrocellulose is the most commonly used propellent powder base. Thenitrocellulose may be used alone or in combination with deterrents suchas dinitrotoluene, dialkyl and diaryl phthalates, etc. Energizers suchas nitroglycerin, aluminum, trinitrotoluene,cyclotrimethyienetr-initramine, ammonium perchlorate, and the like canalso be incorporated with the nitrocellulose or mixed with the surfacecoated powder. Whenever desirable, the nitrocellulose can be replaced inwhole or in part by other gelatinizable polynitro materials such asnitrostarch, polyvinyl nitrate, and similar materials.

While one complete embodiment has been disclosed in detail andmodifications therein suggested, it is to be understood that the examplegiven is for the purpose of illustrating the invention, and not by wayof limitation. It is recognized that those skilled in the art will makeappropriate adjustment of the several variables in the process in orderto adapt it to the peculiarities of any given operation, andconsequently it is to be distinctly understood that the invention is notlimited to the examples given, save as indicated in the appended claims.

Having thus described the invention, what is claimed and desired to besecured by Letters Patent is:

1. A dustless propcllent powder charge consisting essentially ofnon-porous solidified globules of nitrocellulose base powder grainshaving an average diameter of less than about 0.003 inch andsubstantially uniformly surface coated with from about 0.02% to 0.5 ofdioctyl phthalate.

2. A dustless propellent powder charge consisting essentially ofspherical nitrocellulose propellent powder grains having an averagediameter less than about 0.003 inch and coated substantially uniformlywith between about 0.02% and 0.5% of a liquid ester formed by theesterification of a dibasic acid selected from the group consisting ofp-hthalic acid and the dibasic saturated aliphatic acids having between4 and 8 carbon atoms in their alkyl chains with a saturated aliphaticalcohol containing between 6 and 12 carbon atoms in the alkyl chain.

3. In the manufacture of rocket propellent grains the steps comprisingintroducing a water slurry of finely divided nitrocellulose powdergrains to a centrifuge, centrifuging the slurry to remove the watertherefrom, adding an aqueous emulsion of dioctyl phthalate to the powderremaining in the centrifuge, the amount of dioctyl phthalate beingbetween about 0.02 and 0.5% based on the weight of the powder grains,mixing the emulsion with the powder, centrifuging the resultant mixtureto remove liquid therefrom, drying the powder grains, mixing the driedpowder grains with a casting solvent to form a plastisol mixture, andforming the plastisol mixture into grains.

4. 1n the manufacture of rocket propellent grains the steps comprisingintroducing a water slurry of finely divided nitrocellulose powdergrains to a centrifuge, centrifuging the slurry to remove the watertherefrom, adding an aqueous emulsion of dioctyl phthalate to the powderremaining in the centrifuge, the amount of dioctyl phthalate beingbetween about 0.02 and 0.5% based on the weight of the powder grains,mixing the emulsion with the powder, centrifuging the resultant mixtureto remove liquid therefrom, and drying the powder grains.

5. A method of preparing fine diameter spherical particles ofnitrocellulose prope'llent powder for casting into a rocket graincomprising mixing the powder grains with from about 0.02% to about 0.5%of a liquid alkyl diester formed by the esterification of a dibasic acidselected from the group consisting of phthalic acid and the dibasicsaturated aliphatic acids having between 4 and 8 carbon atoms in theiralkyl chains with a saturated aliphatic alcohol containing between 6 and12 carbon atoms in the alkyl chain, whereby a substantially uniformcoating of said ester is formed on the surface of said grains.

6. A dustless astatic propellent powder charge consisting essentially ofnitrocellulose base propellent powder grains provided with asubstantially uniform surface coating of from about 0.02 to 0.5% byweight of a liquid ester formed by the esterification of a dibasic acidselected from the group consisting of phthalic acid and the dibasicsaturated aliphatic acids having between 4 and 8 carbon atoms in theiralkyl chains with a saturated aliphatic alcohol containing between 6 and12 carbon atoms in the alkyl chain, the interior of the grains beingsubstantially devoid of the ester.

References Cited in the file of this patent UNITED STATES PATENTS2,235,298 Olsen Mar. 18, 1941 2,916,775 ONeill Dec. 15, 1959 2,917,379Ryker Dec. 15. 1959

6. A DUSTLESS ASTATIC PROPELLENT POWDER CHARGE CONSISTING ESSENTIALLY OFNITROCELLULOSE BASE PROPELLENT POWDER GRAINS PROVIDED WITH ASUBSTANTIALLY UNIFORM SURFACE COATING OF FROM ABOUT 0.02 TO 0.05% BYWEIGHT OF LIQUID ESTER FORMED BY THE ESTERIFICATION OF A DIBASIC ACIDSELECTED FROM THE GROUP CONSISTING OF PHTHALIC ACID AND THE DIBASICSATURATED ALIPHATIC ACIDS HAVING BETWEEN 4 AND 8 CARBON ATOMS IN THEIRALKYL CHAINS WITH A SATURATED ALIPHATIC ALCOHOL CONTAINING BETWEEN 6 AND12 CARBON ATOMS IN THE ALKYL CHAIN, THE INTERIOR OF THE GRAINS BEINGSUBSTANTIALLY DEVOID OF THE ESTER.